Effects of Joint Clearance on Motion Accuracy of Robotic Manipulators

Robots have been introduced in many industrial and medical areas where high accuracy, repeatability, and operation stability are desired. These are key features for robots. Some error sources in robots originate from assemblage, servo actuator resolution, reducer backlash, and joint clearances [1]. These errors reduce the accuracy of the robot and must be controlled to ensure the quality of the desired movement. In this regard, increased importance has been given to the accuracy of robots through various contributions in the relevant literature [2] to [5]. In comparison to machine tools, industrial robots are flexible and relatively cheaper in terms of cost. At the same time, such robots are susceptible to errors from many sources due to their serial structure. To ensure the positioning accuracy of a robot end effector as well as to reduce the manufacturing cost of the robot, it is necessary to quantify the influence of the uncertain factors and optimally allocate the tolerances. A novel and simple approach to identify the positional and directional errors due to the joint clearance of linkages and manipulators based on a geometrical model was introduced [6]. A general probability density function of the endpoint of planar robots with joint clearance was established to derive the distribution functions for any position tolerance zone and any joint distribution type [7]. Some errors arising from link stiffness and clearances were considered to predict the accuracy of the parallel devices [8]. By considering the positional and directional errors of the robot hand and the manufacturing cost, the optimal allocation of joint tolerances was investigated. Interval analysis was used to predict the errors in the manipulator performance [9]. Singularity analysis and modelling of the joint clearance effects on the parallel robot’s accuracy were conducted. An analytical model was presented to easily predict the pose error for a given external load, a nominal pose and the structural parameters [3]. A procedure to calculate the positional error in parallel manipulators due to both clearances and elastic deformations was proposed [10]. For analysing the location of the discontinuities, a methodology was presented and the advantages of approach were analysed using a 5R planar mechanism [11]. The effect of joint flexibility on the dynamic performance of a serial spatial robot arm with rigid links was studied by using three developed models [12]. A novel method based on trajectory planning to avoid detachment of the joint elements of a manipulator with clearances was presented. An improved detachment criterion for the different joint types was proposed [13]. The clearance effects on an industrial robot were studied during 2D welding operations. The kinematics and Effects of Joint Clearance on the Motion Accuracy of Robotic Manipulators Erkaya, S. Selçuk Erkaya* Erciyes University, Engineering Faculty, Mechatronics Engineering Department, Turkey